Method of making ultraviolet-transmitting high-silica glasses



Patented Oct. 7, 1952 METHOD 'OF ULTRAVIOLET- TRANSMIT-TINGHIGH-SILICA GLASSES Martin E ration of N'e'wYork A .Nprdberg, Gorning,-N. CorningGlass Works, Corning,

-assignor to N. Y., a corpo- No Drawing. Application April 21, 1950,

Serial No. 157,404

, J9 Claims. 1

This invention relates to the treatment of glasses of the type disclosed in :Patents Nos. 2,106,744 and 2,221,709, and particularly to a method of improving thepropertiesand useiulness of such glasses for ultraviolet-emitting lamp envelopes.

According to such patents, an article composed of a glass containing over 94% S102 can be produced by melting and shaping aparticular easily meltable glass of lower silica content and extracting therefrom soluble, nonsiliceous constituents by leaching to leave a higlysiliceous structure retaining its original shape and-having a multiplicity of intercommunicating submicroscopic pores, which porous-glass can be-consolidated to a nonporous condition-by heating.

On account of its high annealing point (on the order of 900 (3.), such high-silica glass should be particularly adapted ior themanufac-ture of tubes for use as envelopes for .medium--,press1 re, mercury arc lamps; but such use is attended-by difliculties in practice. The usual preliminary baking out of an envelope of such glass at the temperature (400? to 800 C.) customarily employed to eliminate .gas from the interior oftubing to be utilized for such ,purposeis not entirely effective, with the result that a continuous, small but objectionableevolution of watervapor and other gas from the inner surface of such an envelope occurs during operation of a lamp made therefrom, whereby the useful life of the lamp is shortened. Moreover, any-trace-of iron remaining in the porous glass-following leaching, is in part-present in the ferric stateafter consolidation and causes an undesirabledegree'of absorption of ultraviolet radiations, an objectionable factor in lamps intended for ultraviolet emission.

In my copending application serial No. v657,- 916, filed March 28, 1946, now Patent 2,505,001, I have described and claimed amethod for treating tubes of such porous high-silicaglass, during consolidation of the glass, to reducethe iron to the less absorptive ferrous state and toeliminate water vapor and other dissolved'gas from the glass. Such method comprises heating the porous glass tubes in an atmosphereof, a hydrogencontaining reducinggas to reduce the iron and thereafter heating the tubes to the consolidating temperature in a substantialvacuum fora time sufficient to consolidate the glass to a nonporous condition. As a result of such treatment the ultraviolet transmission of the glassjis improved andits water and gas content is decreased to a negligible value thereby increasing the cftubes, and warping and ficiencyand effective life of lamps made from h ub In -practicing such .method, however, certain practical difiicultiesare encountered. For example, such porous: glasstubes, when heated at a temperature-sufiicient to close the pores in the glass, tend to warp and flatten to an oval shape. When such tubes are so heated under atniosvpheric pressure, it is readily possible to irotate them about a longitudinal'ax'is and thus avoid such warpingand flattening. Whensuchtubes are soheated in a vacuum in accordance {with the method of my copending application, however, it is impossible .to satisfactorily rotate them without the use of very special equipment Which tends to renderithe operationprohibitive inlcost.

Consequently, tubes produced by such vacuumfiring method which are warped or out of round .mustsubsequently be reheated under'athiospheric pressure in a rotating furnace at extra ex- .pense, andeven then they may not be satisfactory in contour,

I have now discovered that the difiiculties and disadvantages of-Jsuch vacuum-firing procedure can be practically eliminated with substantial avoidance of Warping and flattening of the tubes,

.while at the same timesatisfactorily decreasing the water andgas content of theglass', by a specialtreatment of. suchtubes under anhydrous conditions While they alefbeing heated-t0 effect consolidation of the porousglas's."

, The present invention comprises heating the porous glass tubes in an atmosphere of a reducing gas, preferably an atmosphere containing hydrogen, to a temperature insumcie'nt'to effect s'ub stantial consolidation, and I thereafter heating them, advantageously jatT'substaiitially" atmospheric pressure, in an anhydrous atmosphere which is free of any water-forming constituent and which will not cause substantial oxidation of dation temperature, fora-time and; ata temperature insufiicientto fuse the glassj but suflicient to consolidate it to a nonporous condition. --l;3y such procedure not only is itpossible to effect substantial reduction of; any ferric iron present in the glass and satisfactpry removal of water and gas therefrom, but, also conventional, equipment can be used for consolidation of the porous glass flattening of the tubes ferrous iron present in-the gassa the consolican thusbe avoided. V V U In the practice of the ,present procedure, the

porous glass tubes are advantageously}subjected to an initial dehy elratlcn treatment tor; elimination of water looselyheldwithin the pores.

dehydration is effected by heating the tubes at a temperature ranging from about 600 to 900 C. Heating-up should be sufficiently slow to avoid rupture of the glass by too rapid drying.

The initially dehydrated glass tubes are then heated in the reducing atmosphere at a temperature ranging from about 900 to 1050 C. for reduction of any ferric iron that may be present. Although use of the reducing atmosphere can be begun at a temperature below 900 C., no particular advantage is derived thereby. The use of the reducing atmosphere at temperatures above about 1050 C. is not particularly desirable, since consolidation of the porous glass already begins to take place to an appreciable extent at such point.

For this reason replacement of the reducing I atmosphere by the anhydrous atmosphere is preferably undertaken at temperatures ranging between 950 and 1050 C., and the tubes are desirably maintained at this temperature for a time sufficient to effect substantial elimination of any residual water and gas before any appreciable consolidation of the porous glass takes place. Complete consolidation of the porous glass is then effected in the presence of such anhydrous atmosphere by heating the glass to a temperature in the range between about 1150 and 1250 C. with further elimination of water and gas from the glass.

The anhydrous atmosphere may be composed of a gas selected from the group consisting of nitrogen, carbon dioxide, carbon monoxide, sulfur dioxide, helium, neon and argon. Each of these gases is suitable for the present purpose in that water cannot be formed therefrom and it does not cause any appreciable reoxidation of the ferrous iron within the range of temperatures at which consolidation of the porous glass is effected.

In a preferred manner of carrying out the invention, tubes composed of the porous glass produced in accordance with the methods described in Patents Nos. 2,106,744 and 2,221,709 are advantageously initially slowly heated and held at 600 to 650 C. for 2 hours, or at higher temperatures up to 900 C. for correspondingly shorter times to remove most of the water from the glass. Such preliminary heating preferably is done in a preheating furnace.

The porous glass tubes are then transferred to a rotatable muffle furnace heated to 900 to 1050 C., and the air within the muffle is displaced by a reducing gas at substantially atmospheric pressure in order to insure reduction of any ferric iron present in the glass to ferrous iron. The tubes are desirably maintained in this temperature range for A to 1 hour. As the reducing gas, hydrogen or a nonexplosive mixture consisting of 92% nitrogen and 8% bydrogen is suitable. A cheaper reducing gas is natural gas that has been partially burned and cracked, which gas can be produced in the known manner by passage of natural gas through a semicombustion chamber.

The atmosphere of reducing gas in the furnace is thereafter displaced by anhydrous nitrogen at substantially atmospheric pressure, and the temperature is held at 1000 to 1050 C. for /2 to 4 hours to insure satisfactory removal of water and gas from the glass. The temperature thereafter is raised to 1200 to 1225 C. and is maintained there for at least minutes to consolidate the porous glass to the nonporous condition. The furnace is-then cooled. The flow of anhydrous nitrogen into the furnace is preferably continued until the temperature has cooled to about 1000 C. in order to prevent reoxidation of the ferrous oxide, and the gas is then shut off. The glass may then be removed from the furnace.

It'is preferred to use nitrogen as the anhydrous atmosphere for the consolidation treatment but carbon dioxide, carbon monoxide or sulfur dioxide may be substituted therefor. The inert gases helium, neon or argon may also be used with good results but at an increase in cost. Hydrogen, although nonoxidizing, is not suitable because, at the temperatures employed in the consolidating step, water is continuously formed through an apparent reaction of the hydrogen with some ingredient in the glass. Hence its use in lieu of nitrogen does not effect the requisite dehydration of the glass.

If desired, the entire heat treatment can be carried out in a single furnace, the reducing gas being introduced at 900 to 1000 C. until reduction is complete and the anhydrous nitrogen being introduced at about 1000 C. The temperature can be increased either gradually or stepwise.

Although the present method has been described in connection with the treatment of porous glass tubes, other articles of the same type of glass may also be advantageously treated by such method.

I claim: I

1. The process of treating an article composed of a glass containing over 94% S102 and having throughout its mass a multiplicity of submicroscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes heating the article in a hydrogen-containing reducing atmosphere to a temperature sufficient to cause substantial reduction of the ferric iron present therein but insufficient to effect substantial consolidation of the glass and thereafter heating the article in an anhydrous atmosphere composed of a gas selected from the group consisting of nitrogen, carbon dioxide, carbon monoxide, sulfur dioxide, helium, neon, and argon to a temperature insufficient to fuse the glass but sufficient to consolidate it to a nonporous condition until consolidation is complete.

2. The process of treating an article composed of a glass containing over 94% S102 and having throughout its mass a multiplicity of submicroscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes heating the article in a hydrogencontaining reducing atmosphere to a temperature sufficient to cause substantial reduction of the ferric iron present therein but insuiiicient to effect substantial consolidation of the glass and thereafter heating the article in an anhydrous atmosphere of nitrogen to a temperature insufficient to fuse the glass but sufficient to consolidate it to a nonporous condition until consolidation is complete.

3. The process of treating an article composed of a glass containing over 94% SiOz and having throughout its mass a multiplicity of submicroscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes heating the article in a hydrogen-containing reducing atmosphere to a temperature sufficient to cause substantia1 reduction of the ferric iron present therein but insufficient to effect substantial consolidation of the glass and effect substantial consolidation of the glass and thereafter heating the article in an anhydrous atmosphere of carbon monoxide to a temperature insufficient to fuse the glass but sufficient to consolidate it to a nonporous condition until consolidation is complete.

5. The process of treating an article composed of a glass containing over 94% S102 and having throughout its mass a multiplicity of submicroscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes heating the article in a hydrogen-com taining reducing atmosphere to a temperature sufficient to cause substantial reduction of the ferric iron present therein but insufiicient to effect, substantial consolidation of the glass and thereafter heating the article in an anhydrous atmosphere of sulfur dioxide to a temperature insufficient to fuse the glass but sufficient to consolidate it to a nonporous condition until consolidation is complete.

6. The process of treating an article composed of a glass containing over 94% S102 and having throughout its mass a multiplicity of submicroscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes heating the article in a hydrogen-containing reducing atmosphere to a temperature suflicient to cause substantial reduction of the ferric iron present therein but insufficient to effect substantial consolidation of the glass and thereafter heating the article in an anhydrous atmosphere of helium to a temperature insufficient to fuse the glass but sufficient to consolidate it to a nonporous condition until consolidation is complete.

7. The process of treating an article composed of a glass containing over 94% SiOz and having throughout its mass a multiplicity of submicroscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes heating the article in a hydrogen-containing reducing atmosphere within the range 900 to 1050 0., replacing the reducing atmosphere with an anhydrous atmosphere composed of a gas selected from the group consisting of ni- 6 trogen, carbon dioxide, carbon monoxide, sulfur dioxide, helium, neon and argon, maintaining the article in such anhydrous atmosphere within the range of 950 to 1050 C. 1208119, stantially eliminate any residual water and gas from the glass,and thereafter heating the'article within the range 1-150 to 1250 C. in such anhydrous atmosphere until the glass is consolidated to a nonporous condition.

8. The process of treating an article composed of a glass containing over 94% S102 and having throughout its mass a multiplicity of submicroscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes slowly heating the article to a temperature within the range 600 to 900 C. to dehydrate the glass without rupture, then heating it within the range 900 to 1050 C. in a hydrogen-containing reducing atmosphere, replacing the reducing atmosphere with an anhydrous atmosphere of nitrogen, maintaining the article in such anhydrous nitrogen atmosphere within the range 950 to 1050 C. to subsubstantially eliminate any residual water and gas from the glass, and thereafter heating it Within the range 1150 to 1250 C. in such anhydrous nitrogen atmosphere until the glass is consolidated to a nonporous condition.

9. The process of treating an article composed of a glass containing over 94% S102 and having throughout its mass a multiplicity of submicrcscopic intercommunicating pores, said glass containing a trace of ferric iron, which includes slowly heating the article to a temperature within the range 600 to 900 C. to dehydrate the glass without rupture, then heating it for A to 1 hour within the range 900 to 1050 C. in an atmosphere of a hydrogen-containing reducing gas which contains hydrogen, replacing the hydrogen-containing gas with an anhydrous atmosphere of nitrogen, and thereafter heating the article in'the anhydrous nitrogen atmosphere for A to 4 hours within the range 1000 to 1050, C. to substantially eliminate any residual water and gas from the glass and then for at least 10 minutes within the range 1200 to 1225 C. to consolidate the glass to a nonporous condition.

MARTIN E. NORDBERG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. THE PROCESS OF TREATING AN ARTICLE COMPOSED OF A GLASS CONTAINING OVER 94% OF SIO2 AND HAVING THROUGHOUT ITS MASS A MULTIPLICITY OF SUBMICROSCOPIC INTERCOMMUNICATING PORES, SAID GLASS CONTAINING A TRACE OF FERRIC IRON, WHICH INCLUDES HEATING THE ARTICLE IN A HYDROGEN-CONTAINING REDUCING ATMOSPHERE TO A TEMPERATURE SUFFICIENT TO CAUSE SUBSTANTIAL REDUCATION OF THE FERRIC IRON PRESENT THEREIN BUT INSUFFICIENT TO EFFECT SUBSTANTIAL CONSOLIDATION OF THE GLASS AND THEREAFTER HEATING THE ARTICLE IN AN ANHYDROUS ATMOSPHERE COMPOSED OF A GAS SELECTED FROM THE GROUP CONSISTING OF NITROGEN, CARBON DIOXIDE, CARBON MONOXIDE, SULFUR DIOXIDE, HELIUM, NEON AND ARGON TO A TEMPERATURE INSUFFICIENT TO FUSE THE GLASS BUT SUFFICIENT TO CONSOLIDATE IT TO A NONPOROUS CONDITION UNTIL CONSOLIDATION IS COMPLETE. 